These steps were worked out by Melvin Calvin and his colleagues at the University of California and, for this reason, are named the Calvin cycle.

Their Experiment

Apparatus used to follow the fate of 14CO2 in the dark reactions. The algal suspension is placed in the "lollypop", supplied with 14CO2, and illuminated. The dark reactions are halted by draining the contents of the lollypop into a flask of hot alcohol. (Courtesy of Dr. James A. Bassham.)

The experimental apparatus is shown at the right. After various intervals of illumination, a suspension of unicellular algae is inactivated and the contents of the cells extracted. The compounds in a drop of the extract are then separated by paper chromatography.

The identity of each substance may be determined simply by comparing its position with the positions occupied by known substances under the same conditions. Or, a fragment containing the spot can be cut from the sheet and chemically analyzed.

To determine which, if any, of the substances separated on the chromatogram are radioactive, a sheet of X-ray film is placed next to the chromatogram. If dark spots appear on the film (because of radiation emitted by the 14C atoms), their position can be correlated with the positions of the chemicals in the chromatogram. Using this technique of autoradiography, Calvin found that 14C turned up in glucose molecules within 30 seconds after the start of photosynthesis. When he permitted photosynthesis to proceed for only 5 seconds, however, the radioactivity was concentrated in several other, smaller, molecules.

The dark spots show the radioactive compounds produced after 10 secs (left) and 2 minutes (right) of photosynthesis by the green alga Scenedesmus. The alga was supplied with carbon dioxide labeled with 14C, a radioactive isotope of carbon.
At 10 seconds, most of the radioactivity is found in 3-phosphoglyceric acid ("P-Glyceric").
At 2 minutes, phosphorylated 6-carbon sugars (glucose and fructose) have been synthesized as well as a number of amino acids.
The small rectangle and circle (lower right-hand corners) mark the spots where the cell extract was applied. (Courtesy of Dr. James A. Bassham.)